This invention relates to a pneumatic servomotor used for assisting in the braking of a motor vehicle.
More particularly, the present invention relates to a pneumatic servomotor for an assisted braking of a motor vehicle, of the type comprising a rigid casing, in which a transverse partition wall is movable, thus defining in an airtight manner a front chamber under a first engine negative pressure, and a rear chamber under a second pressure, varying between the engine negative pressure and the atmospheric pressure; of the type including a moving piston, integral with the moving partition wall and travelling with the latter, and a control rod, moving inside the piston, in a selective manner as a function of an axial input force, exerted in the forward direction against a return force applied to the rod by a return spring; of the type according to which the control rod is biased towards either an intermediate actuation position or an end actuation position, resulting from the applying of the input force at a determined high speed; of the type comprising a plunger, arranged at the front part of the control rod inside the piston, and a three-way valve, including at least an annular seat borne by a rear section of the plunger, and capable of varying the second pressure, prevailing within the rear chamber, particularly by connecting the front chamber with the rear chamber when the control rod is in the rest position, or by gradually connecting the rear chamber with the atmospheric pressure when the control rod is actuated; of the type in which, in the end actuation position of the control rod, a finger, which is slidingly fitted on the front end of the plunger, is biased by the plunger into contact with a reaction disk, integral with the moving piston, so as to transmit the reaction force of the moving piston to the plunger and to the control rod; and of the type comprising a unidirectional clutch device, which includes a coaxial sleeve, sliding on the plunger, and a substantially ring-shaped key, arranged with a given clearance around the sleeve and capable of being driven by the moving piston, when the input force is applied at the determined speed, so as to rotate on a generally transverse axis in order to cooperate with the periphery of the sleeve and lock it in an end front axial position, in which a front end annular bearing surface of the sleeve locks the finger, independently of the plunger and of the control rod.
In a well-known manner, such a design is most suitable as regards safety, in the case of an emergency braking situation.
As a matter of fact, a conventional servomotor comprises neither a finger nor a unidirectional clutch device for the finger. Thus the finger is likely to bias directly the reaction disk, integral with the rear face of the moving piston.
In a full-braking situation, in which case a maximum braking force is applied on the control rod, the actuation of the control rod causes the finger-forming plunger to be actuated, which fact results in the maximum opening of the three-way valve and, therefore, the rear chamber is subjected to the atmospheric pressure. Thus, the moving partition wall travels forward and the end of the plunger contacts the reaction disk, integral with the rear face of the moving piston.
Therefore, the force, which is applied onto the moving piston when the control rod reaches the end of its stroke, results from the assisting force, arising from the pressure difference between each side of the moving partition wall, and from the force exerted by the finger-forming plunger onto said moving piston. Besides, the driver feels the braking reaction force, which is transmitted from the moving piston to the plunger, through the reaction disk.
As a matter of fact, it has been established that quite a number of drivers, when confronted with an emergency braking situation, underestimated the risks actually incurred and, after having jammed the brakes on, would somewhat release the braking force at the very time when a considerable force should have been maintained in order to avoid an accident.
In the case of a full-braking situation, accompanied by the swift travel of the control rod, the plunger may touch the reaction disk and therefore give the driver the feeling of a maximum braking action even before the pressure difference between the front and rear chambers actually reaches its maximum value, which may lead the driver to release the braking force even though it should be maintained so as to profit by the maximum braking force.
A servomotor, like that of the above-described type, makes it possible to eliminate such a disadvantage, in that the plunger is locked into contact with the reaction disk, by means of the sleeve, which results in a maximum force being maintained on the rear face of the moving piston, even though the driver may have released the braking force in part.
Yet, the servomotor of the previously described type has She disadvantage of fixing the sleeve in an axial position but in a rather inaccurate manner.
As a matter of fact, when the key locks the sleeve simply by wedging, that is through a very small contact area, not to say an almost pinpoint one, between an edge of a circular opening made in the key and the periphery of the sleeve, the sleeve is likely to slip when the key comes into contact with it.
In order to cope with said difficulty, the present invention provides reliable sleeve-securing means.
Therefore, it is the object of the present invention to provide a servomotor of the above described type, characterised in that the unidirectional clutch device comprises at least one locking transverse face for the sleeve, said face being axially directed rearwards so as to form a stop for a peg, provided on the key and extending radially towards the sleeve, for an axial indexing of the end locking position of the sleeve.
According to other features of this invention:
the sleeve has a cylindrical periphery, with a substantially constant diameter and it comprises a radial groove, a shoulder-forming front transverse face of which constitutes the locking transverse face;
the groove exhibits a substantially truncated-cone-shaped profile, rearwardly of the shoulder-forming front transverse face;
the peg has the shape of a concave truncated-cone-shaped angular sector, which is complementary to the truncated-cone-shaped profile of the groove in the sleeve;
the key has, in an axial sectional view, the shape of a tee, the vertical branch of which is substantially radially directed and traversed by the sleeve, whereas its horizontal branch, which is substantially axially directed, is received, without any axial clearance, between two opposite walls of a cavity traversing the piston, perpendicularly to its axis, so as to allow but a rocking motion of the key inside the cavity;
the front horizontal half-branch of the tee key has a face which radially faces the sleeve and from which the peg protrudes;
the front end of the front half-branch of the tee key is shaped into a convex arc of a cylinder in the direction of an axis, orthogonal to the piston axis, so as to bear against a front transverse wall of the cavity;
the rear end of the rear half-branch of the tee key is shaped into a convex arc of a cylinder in the direction of an axis, orthogonal to the piston axis, so as to cooperate with a rear transverse wall of the cavity;
the tee key is resiliently biased against the transverse wall of the cavity by a compression spring, arranged between the front transverse wall of the cavity and a centering spigot, provided on the tee key and protruding from a part of the vertical branch of the tee, radially on the opposite side relatively to the peg.